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  webpack-模块打包工具
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      <a href="/2021/05/31/webpack-%E6%A8%A1%E5%9D%97%E6%89%93%E5%8C%85%E5%B7%A5%E5%85%B7/" class="article-date">
  <time datetime="2021-05-31T05:34:13.000Z" itemprop="datePublished">2021-05-31</time>
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  <p><code>webpack</code> 是前端模块常用的打包工具，由于其生态完整，在插件的配合下，功能强大。所以在前端的项目中，在生产环境和开发环境中会通过不同的插件配置，帮助开发者搭建良好的开发体系。</p>
<p><code>webpack</code> 与 <code>grunt、gulp</code> 等构件工具的不同：</p>
<ol>
<li><code>webpack</code> 是一个模块打包工具。将多个文件打包成一个文件，减少生产环境上资源的请求次数</li>
<li><code>grunt/gulp</code> 是自动化构建工具。主要用于对 <code>js/css/html</code> 等资源转换成浏览器可加载的资源</li>
</ol>
<p><code>webpack</code> 工具处理的任务：</p>
<ol>
<li><code>打包文件</code>：将多个有关联的文件打包形成一个文件，减少生产环境中的资源请求次数</li>
<li><code>加载不同类型的资源</code>：<code>webpack</code> 可通过配合使用 <code>loader</code> 实现不同类型文件的加载</li>
<li><code>对资源进行压缩、清除、复制、tree-shaking 等操作</code>：<code>webpack</code> 可配合 <code>plugin</code> 实现不同的操作要求</li>
</ol>
<span id="more"></span>
<h2 id="1-ES-Module-和-CommonJS"><a href="#1-ES-Module-和-CommonJS" class="headerlink" title="1. ES Module 和 CommonJS"></a>1. <code>ES Module</code> 和 <code>CommonJS</code></h2><p><code>CommonJS</code> 是 <code>nodejs</code> 中的模块加载规则。<code>ES Module</code> 是 es6 推出的模块加载规则。俩者之前存在的差异：</p>
<ol>
<li><code>语法不同</code>。<code>CommonJS</code> 中采用 <code>require/module.exports</code> 实现模块的导入导出；<code>ES Module</code> 采用 <code>import/export</code> 实现模块的导入导出。</li>
<li><code>加载资源模式不同</code>。<code>CommonJS</code> 加载模块采用的是同步模式，<code>ES Module</code> 采用的是异步模式。</li>
<li><code>加载时机不同</code>。<code>CommonJS</code> 可以在代码执行的过程中动态加载模块资源，<code>ES Module</code> 加载资源采用静态方式加载。</li>
<li><code>es module 模块中可通过 import 加载 commonjs 的模块</code>。commonjs 导出的是一个 default 资源。</li>
</ol>
<h2 id="2-ES-Module-特别注意点"><a href="#2-ES-Module-特别注意点" class="headerlink" title="2. ES Module 特别注意点"></a>2. <code>ES Module</code> 特别注意点</h2><ol>
<li><code>export 导出</code><ol>
<li><code>export default xxx</code> 默认导出变量 xxx</li>
<li><code>export &#123;&#125;</code> 导出一系列的变量 xxx</li>
<li><code>export</code> 导出的是变量的引用，而非变量的值</li>
</ol>
</li>
<li><code>import 导入</code><ol>
<li>导入的文件路径需要完整，文件后缀和 index.js 文件名不可省略</li>
<li><code>export default xxx</code> 导出的变量，通过 <code>import xxx from &#39;xxx&#39;</code> 实现导入</li>
<li><code>export &#123; a, b &#125;</code> 导出的变量，通过 <code>import &#123;a, b&#125; from &#39;xxx&#39;</code> 实现多个变量的导入。改方式的导入语法和对象的解构一模一样，但是它并不是对象的解构，是 <code>es module</code> 约定的导入语法</li>
<li><code>import</code> 导入的变量只能被以常量使用，即只能读取，不可更改</li>
</ol>
</li>
</ol>
<h2 id="3-webpack-模块加载"><a href="#3-webpack-模块加载" class="headerlink" title="3. webpack 模块加载"></a>3. <code>webpack</code> 模块加载</h2><ol>
<li><code>commonjs 模块内可加载 es module</code> (原生中是不可以加载的)</li>
<li><code>es module 模块内可加载 commonjs</code></li>
</ol>
<p>webpack 在打包的时候会定义自己的模块加载方法 <code>__webpack_require__</code>，用于加载不同模式的模块导出数据。</p>
<h2 id="4-webpack-中的配置"><a href="#4-webpack-中的配置" class="headerlink" title="4. webpack 中的配置"></a>4. webpack 中的配置</h2><ol>
<li><p>加载指定配置文件</p>
<p> 通过 <code>--config</code> 指定 webpack 打包时加载的配置文件</p>
</li>
<li><p><code>loader</code> 机制</p>
<p> 通过配置 loader 加载不同的资源。webpack 默认支持 js 资源的加载，对于 css/image 等资源需要配合 loader 实现加载。<code>多个 loader 组合使用时，从后向前加载</code>。</p>
</li>
<li><p>tree-shaking</p>
<p> 因为 tree-shaking 是针对静态结构进行分析，只有 import 和 export 是静态的导入和导出。</p>
<ol>
<li>通过 usedExports 实现标记枯树叶</li>
<li></li>
</ol>
</li>
<li><p>代码分割</p>
<ol>
<li>通过 <code>import() 实现文件的动态导入</code>，其原理即是<code>实时加载 script 标签</code></li>
</ol>
</li>
<li></li>
</ol>
 
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